A server can have multiple IP addresses, even if it only has 1 NIC.
In fact, every host has at least 2 IP addresses: The host's Layer 3 identifier, and the host's loopback address.
A service can choose to bind to the same port for all addresses (usually denoted by 0.0.0.0:<port> in case of IPv4), or just a certain address:port tuple.
For a made up example:
Let's say I have a (dev) web server + Varnish. The server has 3 addresses (but 1 NIC):
- 192.168.123.5
- 192.168.123.19
- 127.x.x.x (loopback)
I have explicitly configured the web server to bind to 192.168.123.19:80 and 127.0.0.1:80. The former to allow access from other endpoints in the LAN, the latter to receive redirected requests from Varnish.
The Varnish service is explicitly configured to bind to only 192.168.123.5:80, so all requests coming to that IP address goes through Varnish first before being redirected to 127.0.0.1:80 to be handled by the actual web server.
This way, if I receive errors accessing http://192.168.123.5, I can switch over to http://192.168.123.19 and determine if the error is due to Varnish or due to the web server itself.
Because the server has multiple addresses, every TCP connection needs to be recorded using a 4-tuple of (source.address, source.port, dest.address, dest.port) to ensure that the right response goes through the right connection.
